1. Field of the Invention
The present invention relates generally to bandpass filters, and more particularly, to a switchable bandpass filter having stepped-impedance resonators loaded with diodes.
2. Description of Related Art
A microwave switch is one of the most dominant building blocks in a radio-frequency (RF) front-end for time-division duplexing (TDD) communication systems. Recently, several works using passive field-effect transistors (FET) or p-i-n diodes have been reported for microwave and millimeter-wave transceiver applications (referring to F. J. Huang et al., “A 0.5 μm CMOS T/R switch for 900-MHz wireless applications”, IEEE J. Solid-State Circuits, vol. 36, no. 3, pp. 486-492, March 2001; C. Tinella et al., “A high-performance CMOS-SOI antenna switch for the 2.5-5-GHz band”, IEEE J. Solid-State Circuits, vol. 38, no. 7, pp. 1279-1283, July 2003; Z. Li et al., “15-GHz fully integrated nMOS switches in a 0.13-μm CMOS process”, IEEE J. Solid-State Circuits, vol. 40, no. 11, pp. 2323-2328, November 2005; J. Kim et al., “A high-performance 40-85 GHz MMIC SPDT switch using FET-integrated transmission line structure”, IEEE Microw. Wireless Compon. Lett., vol. 13, no. 12, pp. 505-507, December 2003; K. Y. Lin et al., “Millimeter-wave MMIC passive HEMT switch using traveling-wave concept”, IEEE Trans. Microw. Theory Tech., vol. 52, no. 8, pp. 1798-1808, August 2004). Most of these switches are based on wideband design, implying that their operating bandwidths are over 50% and cannot provide sharp cutoff outside the operating band. Such a wideband switch shows poor band selectivity for system applications. Therefore, a bandpass filter is needed to be cascaded with a switch to reject out-of-band signals. Planar filters are popular in millimeter-wave filter designs because they are easily fabricated using printed circuit technology and integrated with other circuit components. However, conventional design of planar filters suffers from spurious responses in the upper stopband due to the nature of distributed elements (referring to S. B. Cohn, “Parallel coupled transmission-line resonator filters”, IRE Trans. Microw. Theory Tech., vol. MTT-6, no. 2, pp. 223-231, April 1958; E. G. Cristal et al. “Hairpin-line and hybrid hairpin-line/half-wave parallel-coupled-line filters”, IEEE Trans. Microw. Theory Tech., vol. MTT-20, no. 11, pp. 719-728, November 1972). Therefore, several techniques have been proposed to resolve this problem (referring to J. G. Garca et al. “Spurious passband suppression in microstrip coupled line bandpass filters by means of split ring resonators”, IEEE Microw. Wireless Compon. Lett., vol. 14, no. 9, pp. 416-418, September 2004; T. Lopetegi et al., “Microstrip wigglyline bandpass filters with multispurious rejection”, IEEE Microw. Wireless Compon. Lett., vol. 14, no. 11, pp. 531-533, November 2004; K. F. Chang et al., “Miniaturized cross-coupled filter with second and third spurious responses suppression”, IEEE Microw. Wireless Compon. Lett., vol. 15, no. 2, pp. 122-124, February 2005; P. Cheong et al., “Miniaturized parallel coupled-line bandpass filter with spurious-response suppression”, IEEE Trans. Microw. Theory Tech., vol. 53, no. 5, pp. 1810-1816, May 2005; C. F. Chen et al., “Design of microstrip bandpass filters with multiorder spurious-mode suppression”, IEEE Trans. Microw. Theory Tech., vol. 53, no. 12, pp. 3788-3793, December 2005; S. C. Lin et al., “Wide-stopband microstrip bandpass filters using dissimilar quarter-wavelength stepped-impedance resonators”, IEEE Trans. Microw. Theory Tech., vol. 54, no. 3, pp. 1011-1018, March 2006).
From the above discussion, a switchable bandpass filter that integrates the functions of a bandpass filter and a switch is desired to perform a bandpass filter function with wide stopband extension in the ON state and provide a good isolation while in the OFF state. T. S. Martin et al. develop a ring resonator loaded with a p-i-n diode as a switchable filter (referring to “Theoretical and experimental investigation of novel varactor-tuned switchable microstrip ring resonator circuits”, IEEE Trans. Microw. Theory Tech., vol. 36, no. 12, pp. 1733-1739, December 1988). By mounting the p-i-n diodes across the gap at 90 degrees from the feed point, the odd modes can be switched according to different bias conditions to control the ON and OFF states. However, it occupied a large layout size, and a high-order implementation is difficult. Y. H. Shu et al. present a coplanar waveguide-slotline switchable filer, in which p-i-n diodes are mounted over the end of the open stubs to make the circuit switchable (referring to “Electronically switchable and tunable coplanar waveguide-slotline bandpass filters”, IEEE Trans. Microw. Theory Tech., vol. 39, no. 3, pp. 548-554, March 1991). J. Lee et al. propose a switchable microstrip bandpass filter based on quarter-wavelength short-stub structures (referring to “A bandpass filter-integrated switch using field-effect transistors and its power analysis”, IEEE MTT-S Int. Microw. Symp. Dig., June 2006). The quarter-wavelength resonators were replaced by inductive short-stubs shunt with passive FETs to make it switchable. However, these previously mentioned designs mainly focus on designing the performance around the passbands, meaning that only the ON-state filter response and OFF-state isolation in the vicinity of the center frequency were considered. Consequently, those designs would suffer from unwanted spurious response and narrowband isolation in the ON and OFF states, respectively.